We report here thermal reactions between furan and one of three related triyne substrates. Each triyne is capable of reacting initially in two modes: (i) unimolecular hexadehydro-Diels–Alder (HDDA) reaction or (ii) bimolecular Diels–Alder reaction between one of its alkynes with furan. The relative rates of these initial events are such that two of the substrates react essentially in only one of modes (i) or (ii). The third is intermediate in behavior; its bifurcation is dependent on the concentration of the furan reactant. These results teach, more generally, principles relevant to the design of efficient HDDA-based reaction cascades.